Internet Engineering Task Force (IETF) S. Farrell
Request for Comments: 6920 Trinity College Dublin
Category: Standards Track D. Kutscher
ISSN: 2070-1721 NEC
C. Dannewitz
University of Paderborn
B. Ohlman
A. Keranen
Ericsson
P. Hallam-Baker
Comodo Group Inc.
April 2013
Naming Things with Hashes
Abstract
This document defines a set of ways to identify a thing (a digital
object in this case) using the output from a hash function. It
specifies a new URI scheme for this purpose, a way to map these to
HTTP URLs, and binary and human-speakable formats for these names.
The various formats are designed to support, but not require, a
strong link to the referenced object, such that the referenced object
may be authenticated to the same degree as the reference to it. The
reason for this work is to standardise current uses of hash outputs
in URLs and to support new information-centric applications and other
uses of hash outputs in protocols.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6920.
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RFC 6920 Naming Things with Hashes April 20131. Introduction
Identifiers -- names or locators -- are used in various protocols to
identify resources. In many scenarios, those identifiers contain
values that are obtained from hash functions. Different deployments
have chosen different ways to include the hash function outputs in
their identifiers, resulting in interoperability problems.
This document defines the "Named Information" identifier, which
provides a set of standard ways to use hash function outputs in
names. We begin with a few example uses for various ways to include
hash function output in a name, with the specifics defined later in
this document. Figure 1 shows an example of the Named Information
(ni) URI scheme that this document defines.
ni:///sha-256;UyaQV-Ev4rdLoHyJJWCi11OHfrYv9E1aGQAlMO2X_-Q
Figure 1: Example ni URI
Hash function outputs can be used to ensure uniqueness in terms of
mapping URIs [RFC3986] to a specific resource or to make URIs hard to
guess for security reasons. Since there is no standard way to
interpret those strings today, in general only the creator of the URI
knows how to use the hash function output. Other protocols, such as
application-layer protocols for accessing "smart objects" in
constrained environments, also require more compact (e.g., binary)
forms of such identifiers. In yet other situations, people may have
to speak such values, e.g., in a voice call (see Section 8.3), in
order to confirm the presence or absence of a resource.
As another example, protocols for accessing in-network storage
servers need a way to identify stored resources uniquely and in a
location-independent way so that replicas on different servers can be
accessed by the same name. Also, such applications may require
verification that a resource representation that has been obtained
actually corresponds to the name that was used to request the
resource, i.e., verifying the binding between the data and the name,
which is here termed "name-data integrity".
Similarly, in the context of information-centric networking
[NETINF-ARCHITECTURE] [CCN] and elsewhere, there is value in being
able to compare a presented resource against the URI that was used to
access that resource. If a cryptographically strong comparison
function can be used, then this allows for many forms of in-network
storage, without requiring as much trust in the infrastructure used
to present the resource. The outputs of hash functions can be used
in this manner, if they are presented in a standard way.
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Additional applications might include creating references from web
pages delivered over HTTP/TLS; DNS resource records signed using
DNSSEC or data values embedded in certificates, Certificate
Revocation Lists (CRLs), or other signed data objects.
The Named Identifier can be represented in a number of ways: using
the ni URI scheme that we specifically define for the name (which is
very similar to the "magnet link" that is informally defined in other
protocols [Magnet]), or using other mechanisms also defined herein.
However it is represented, the Named Identifier *names* a resource,
and the mechanism used to dereference the name and to *locate* the
named resource needs to be known by the entity that dereferences it.
Media content-type, alternative locations for retrieval, and other
additional information about a resource named using this scheme can
be provided using a query string. "The Named Information (ni) URI
Scheme: Optional Features" [DECPARAMS] describes specific values that
can be used in such query strings for these various purposes and
other extensions to this basic format specification.
In addition, we define a ".well-known" URL equivalent, a way to
include a hash as a segment of an HTTP URL, a binary format for use
in protocols that require more compact names, and a human-speakable
text form that could be used, e.g., for reading out (parts of) the
name over a voice connection.
Not all uses of these names require use of the full hash output --
truncated hashes can be safely used in some environments. For this
reason, we define a new IANA registry for hash functions to be used
with this specification so as not to mix strong and weak (truncated)
hash algorithms in other protocol registries.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Syntax definitions in this memo are specified according to ABNF
[RFC5234].
2. Hashes Are What Count
This section contains basic considerations related to how we use hash
function outputs that are common to all formats.
When comparing two names of the form defined here, an implementation
MUST only consider the digest algorithm and the digest value, i.e.,
it MUST NOT consider other fields defined below (such as an authority
field from a URI or any parameters). Implementations MUST consider
Farrell, et al. Standards Track [Page 4]

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two hashes identical, regardless of encoding, if the decoded hashes
are based on the same algorithm and have the same length and the same
binary value. In that case, the two names can be treated as
referring to the same thing.
The sha-256 algorithm as specified in [SHA-256] is mandatory to
implement; that is, implementations MUST be able to generate/send and
to accept/process names based on a sha-256 hash. However,
implementations MAY support additional hash algorithms and MAY use
those for specific names, for example, in a constrained environment
where sha-256 is non-optimal or where truncated names are needed to
fit into corresponding protocols (when a higher collision probability
can be tolerated).
Truncated hashes MAY be supported. When a hash value is truncated,
the name MUST indicate this. Therefore, we use different hash
algorithm strings in these cases, such as sha-256-32 for a 32-bit
truncation of a sha-256 output. A 32-bit truncated hash is
essentially useless for security in almost all cases but might be
useful for naming. With current best practices [RFC3766], very few,
if any, applications making use of names with less than 100-bit
hashes will have useful security properties.
When a hash value is truncated to N bits, the leftmost N bits (that
is, the most significant N bits in network byte order) from the
binary representation of the hash value MUST be used as the truncated
value. An example of a 128-bit hash output truncated to 32 bits is
shown in Figure 2.
128-bit hash: 0x265357902fe1b7e2a04b897c6025d7a2
32-bit truncated hash: 0x26535790
Figure 2: Example of Truncated Hash
When the input to the hash algorithm is a public key value, as may be
used by various security protocols, the hash SHOULD be calculated
over the public key in an X.509 SubjectPublicKeyInfo structure
(Section 4.1 of [RFC5280]). This input has been chosen primarily for
compatibility with the DANE TSLA protocol [RFC6698] but also includes
any relevant public key parameters in the hash input, which is
sometimes necessary for security reasons. This does not force use of
X.509 or full compliance with [RFC5280] since formatting any public
key as a SubjectPublicKeyInfo is relatively straightforward and well
supported by libraries.
Any of the formats defined below can be used to represent the
resulting name for a public key.
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Other than in the aforementioned special case where public keys are
used, we do not specify the hash function input here. Other
specifications are expected to define this.
3. Named Information (ni) URI Format
A Named Information (ni) URI consists of the following nine
components:
Scheme Name: The scheme name is 'ni'.
Colon and Slashes: The literal "://"
Authority: The optional authority component may assist applications
in accessing the object named by an ni URI. There is no default
value for the authority field. (See Section 3.2.2 of [RFC3986]
for details.) While ni names with and without an authority differ
syntactically from ni names with different authorities, all three
refer to the same object if and only if the digest algorithm,
length, and value are the same.
One slash: The literal "/"
Digest Algorithm: The name of the digest algorithm, as specified in
the IANA registry defined in Section 9.4 below.
Separator: The literal ";"
Digest Value: The digest value MUST be encoded using the base64url
[RFC4648] encoding, with no "=" padding characters.
Query Parameter separator '?': The query parameter separator acts as
a separator between the digest value and the query parameters (if
specified). For compatibility with Internationalized Resource
Identifiers (IRIs), non-ASCII characters in the query part MUST be
encoded as UTF-8, and the resulting octets MUST be percent-encoded
(see [RFC3986], Section 2.1).
Query Parameters: A "tag=value" list of optional query parameters as
are used with HTTP URLs [RFC2616] with a separator character '&'
between each. For example, "foo=bar&baz=bat".
It is OPTIONAL for implementations to check the integrity of the URI/
resource mapping when sending, receiving, or processing ni URIs.
Escaping of characters follows the rules in RFC 3986. This means
that percent-encoding is used to distinguish between reserved and
unreserved functions of the same character in the same URI component.
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As an example, an ampersand ('&') is used in the query part to
separate attribute-value pairs; therefore, an ampersand in a value
has to be escaped as '%26'. Note that the set of reserved characters
differs for each component. As an example, a slash ('/') does not
have any reserved function in a query part and therefore does not
have to be escaped. However, it can still appear escaped as '%2f' or
'%2F', and implementations have to be able to understand such escaped
forms. Also note that any characters outside those allowed in the
respective URI component have to be escaped.
The Named Information URI adapts the URI definition from the URI
Generic Syntax [RFC3986]. We start with the base URI production:
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
; from RFC 3986
Figure 3: URI Syntax
Then, we adapt that for the Named Information URI:
NI-URI = ni-scheme ":" ni-hier-part [ "?" query ]
; adapted from "URI" in RFC 3986
; query is from RFC 3986, Section 3.4
ni-scheme = "ni"
ni-hier-part = "//" [ authority ] "/" alg-val
; authority is from RFC 3986, Section 3.2
alg-val = alg ";" val
; adapted from "hier-part" in RFC 3986
alg = 1*unreserved
val = 1*unreserved
; unreserved is from RFC 3986, Section 2.3
Figure 4: ni Name Syntax
The "val" field MUST contain the output of base64url encoding (with
no "=" padding characters), the result of applying the hash function
("alg") to its defined input, which defaults to the object bytes that
are expected to be returned when the URI is dereferenced.
Relative ni URIs can occur. In such cases, the algorithm in Section5 of [RFC3986] applies. As an example, in Figure 5, the absolute URI
for "this third document" is "ni://example.com/sha-256-128;...".
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<html>
<head>
<title>ni: relative URI test</title>
<base href="ni://example.com">
</head>
<body>
<p>Please check <a href="sha-256;f4OxZX...">this document</a>.
and <a href="sha-256;UyaQV...">this other document</a>.
and <a href="sha-256-128;...">this third document</a>.
</p>
</body>
</html>
Figure 5: Example HTML with Relative ni URI
The authority field in an ni URI is not quite the same as that from
an HTTP URL, even though the same values (e.g., DNS names) may be
usefully used in both. For an ni URI, the authority does not control
nearly as much of the structure of the "right-hand side" of the URI.
With ni URIs we also define standard query string attributes and, of
course, have a strictly defined way to include the hash value.
Internationalisation of strings within ni names is handled exactly as
for http URIs -- see [RFC2616], Section 3.2.3.
3.1. Content Type Query String Attribute
The semantics of a digest being used to establish a secure reference
from an authenticated source to an external source may be a function
of associated metadata such as the Content Type. The Content Type
"ct" parameter specifies the MIME Content Type of the associated data
as defined in [RFC6838]. See Section 9.5 for the associated IANA
registry for ni parameter names as shown in Figure 6.
Implementations of this specification MUST support parsing the "ct="
query string attribute name.
ni:///sha-256-32;f4OxZQ?ct=text/plain
Figure 6: Example ni URI with Content Type
Protocols making use of ni URIs will need to specify how to verify
name-data integrity for the MIME Content Types that they need to
process and will need to take into account possible Content-Transfer-
Encodings and other aspects of MIME encoding.
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Implementations of this specification SHOULD support name-data
integrity validation for at least the application/octet-stream
Content Type, with no explicit Content-Transfer-Encoding (which is
equivalent to binary). Additional Content Types and Content-
Transfer-Encodings can of course also be supported, but are OPTIONAL.
Note that the hash is calculated after the Content-Transfer-Encoding
is removed so it is applied to the raw data.
If a) the user agent is sensitive to the Content Type and b) the ni
name used has a "ct=" query string attribute and c) the object is
retrieved (from a server) using a protocol that specifies a Content
Type, then, if the two Content Types match, all is well. If, in this
situation, the Content Types do not match, then the client SHOULD
handle that situation as a potential security error. Content Type
matching rules are defined in [RFC2045], Section 5.1.
4. .well-known URI
We define a mapping between URIs following the ni URI scheme and HTTP
[RFC2616] or HTTPS [RFC2818] URLs that makes use of the .well-known
URI [RFC5785] by defining an "ni" suffix (see Section 9).
The HTTP(S) mapping MAY be used in any context where clients with
support for ni URIs are not available.
Since the .well-known name-space is not intended for general
information retrieval, if an application dereferences a
.well-known/ni URL via HTTP(S), then it will often receive a 3xx HTTP
redirection response. A server responding to a request for a
.well-known/ni URL will often therefore return a 3xx response, and a
client sending such a request MUST be able to handle that, as should
any fully compliant HTTP [RFC2616] client.
For an ni name of the form "ni://n-authority/alg;val?query-string"
the corresponding HTTP(S) URL produced by this mapping is
"http://h-authority/.well-known/ni/alg/val?query-string", where
"h-authority" is derived as follows: If the ni name has a specified
authority (i.e., the n-authority is non-empty), then the h-authority
MUST have the same value. If the ni name has no authority specified
(i.e., the n-authority string is empty), a h-authority value MAY be
derived from the application context. For example, if the mapping is
being done in the context of a web page, then the origin [RFC6454]
for that web site can be used. Of course, in general there are no
guarantees that the object named by the ni URI will be available via
the corresponding HTTP(S) URL. But in the case that any data is
returned, the retriever can determine whether or not it is content
that matches the ni URI.
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If an application is presented with an HTTP(S) URL with
"/.well-known/ni/" as the start of its pathname component, then the
reverse mapping to an ni URI either including or excluding the
authority might produce an ni URI that is meaningful. However, there
is no guarantee that this will be the case.
When mapping from an ni URI to a .well-known URL, an implementation
will have to decide between choosing an "http" or "https" URL. If
the object referenced does in fact match the hash in the URL, then
there is arguably no need for additional data integrity, if the ni
URI or .well-known URL was received "securely." However, TLS also
provides confidentiality, so there can still be reasons to use the
"https" URL scheme even in this case. Additionally, web server
policy such as [RFC6797] may dictate that data only be available over
"https". In general, however, whether to use "http" or "https" is
something that needs to be decided by the application.
5. URL Segment Format
Some applications may benefit from using hashes in existing HTTP URLs
or other URLs. To do this, one simply uses the "alg-val" production
from the ni name scheme ABNF, which may be included, for example, in
the pathname, query string, or even fragment components of HTTP URLs
[RFC2616]. In such cases, there is nothing present in the URL that
ensures that a client can depend on compliance with this
specification, so clients MUST NOT assume that any URL with a
pathname component that matches the "alg-val" production was in fact
produced as a result of this specification. That URL might or might
not be related to this specification, only the context will tell.
6. Binary Format
If a more space-efficient version of the name is needed, the
following binary format can be used. The binary format name consists
of two fields: a header and the hash value. The header field defines
how the identifier has been created, and the hash value contains a
(possibly truncated) result of a one-way hash over whatever is being
identified by the hash value. The binary format of a name is shown
in Figure 7.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Res| Suite ID | Hash Value /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... |
+-+-+-+-+-+-+-+-+
Figure 7: Binary Name Format
The Res field is a reserved 2-bit field for future use and MUST be
set to zero for this specification and ignored on receipt.
The hash algorithm and truncation length are specified by the Suite
ID. For maintaining efficient encoding for the binary format, only a
few hash algorithms and truncation lengths are supported. See
Section 9.4 for details.
A hash value that is truncated to 120 bits will result in the overall
name being a 128-bit value, which may be useful for protocols that
can easily use 128-bit identifiers.
7. Human-Speakable (nih) URI Format
Sometimes a resource may need to be referred to via a name in a
format that is easy for humans to read out and less likely to be
ambiguous when heard. This is intended to be usable, for example,
over the phone in order to confirm the (current or future) presence
or absence of a resource. This "confirmation" use-case described
further in Section 8.3 is the main current use-case for Named
Information for Humans (nih) URIs. ("nih" also means "Not Invented
Here", which is clearly false, and therefore worth including
[RFC5513]. :-)
The ni URI format is not well-suited for this, as, for example,
base64url uses both uppercase and lowercase, which can easily cause
confusion. For this particular purpose ("speaking" the value of a
hash output), the more verbose but less ambiguous (when spoken) nih
URI scheme is defined.
The justification for nih being a URI scheme is that it can help a
user agent for the speaker to better display the value or help a
machine to better speak or recognise the value when spoken. We do
not include the query string since there is no way to ensure that its
value might be spoken unambiguously and similarly for the authority,
where, e.g., some internationalised forms of domain name might not be
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easy to speak and comprehend easily. This leaves the hash value as
the only part of the ni URI that we feel can be usefully included.
But since speakers or listeners (or speech recognition) may err, we
also include a checkdigit to catch common errors and allow for the
inclusion of "-" separators to make nih URIs easier to read out.
Fields in nih URIs are separated by a semicolon (;) character. The
first field is a hash algorithm string, as in the ni URI format. The
hash value is represented using lowercase ASCII hex characters; for
example, an octet with the decimal value 58 (0x3A) is encoded as
'3a'. This is the same as base16 encoding as defined in RFC 4648
[RFC4648] except using lowercase letters. Separators ("-"
characters) MAY be interspersed in the hash value in any way to make
those easier to read, typically grouping four or six characters with
a separator between.
The hash value MAY be followed by a semicolon ';' then a checkdigit.
The checkdigit MUST be calculated using Luhn's mod N algorithm (with
N=16) as defined in [ISOIEC7812] (see also [Luhn]). The input to the
calculation is the ASCII hex-encoded hash value (i.e., "sepval" in
the ABNF production below) but with all "-" separator characters
first stripped out. This maps the ASCII hex so that '0'=0, ...'9'=9,
'a'=10, ...'f'=15. None of the other fields, nor any "-" separators,
are input when calculating the checkdigit.
humanname = "nih:" alg-sepval [ ";" checkdigit ]
alg-sepval = alg ";" sepval
sepval = 1*(ahlc / "-")
ahlc = DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
; DIGIT is defined in RFC 5234 and is 0-9
checkdigit = ahlc
Figure 8: Human-Speakable Syntax
For algorithms that have a Suite ID reserved (see Figure 11), the alg
field MAY contain the ID value as an ASCII-encoded decimal number
instead of the hash name string (for example, "3" instead of
"sha-256-120"). Implementations MUST be able to match the decimal ID
values for the algorithms and hash lengths that they support, even if
they do not support the binary format.
There is no such thing as a relative nih URI.
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+-------------------------------------------------------------------+
| URI: |
| ni:///sha-256;UyaQV-Ev4rdLoHyJJWCi11OHfrYv9E1aGQAlMO2X_-Q |
+-------------------------------------------------------------------+
| .well-known URL (split over 2 lines): |
| http://example.com/.well-known/ni/sha256/ |
| UyaQV-Ev4rdLoHyJJWCi11OHfrYv9E1aGQAlMO2X_-Q |
+-------------------------------------------------------------------+
| URL Segment: |
| sha-256;UyaQV-Ev4rdLoHyJJWCi11OHfrYv9E1aGQAlMO2X_-Q |
+-------------------------------------------------------------------+
| Binary name (ASCII hex encoded) with 120-bit truncated hash value |
| which is Suite ID 0x03: |
| 0353 2690 57e1 2fe2 b74b a07c 8925 60a2 |
+-------------------------------------------------------------------+
| Human-speakable form of a name for this key (truncated to 120 bits|
| in length) with checkdigit: |
| nih:sha-256-120;5326-9057-e12f-e2b7-4ba0-7c89-2560-a2;f |
+-------------------------------------------------------------------+
| Human-speakable form of a name for this key (truncated to 32 bits |
| in length) with checkdigit and no "-" separators: |
| nih:sha-256-32;53269057;b |
+-------------------------------------------------------------------+
| Human-speakable form using decimal presentation of the |
| algorithm ID (sha-256-120) with checkdigit: |
| nih:3;532690-57e12f-e2b74b-a07c89-2560a2;f |
+-------------------------------------------------------------------+
Figure 10: Example Names
8.3. nih Usage Example
Alice has set up a server node with an RSA key pair. She uses an ni
URI as the name for the public key that corresponds to the private
key on that box. Alice's node might identify itself using that ni
URI in some protocol.
Bob would like to believe that it's really Alice's node when his node
interacts with the network and asks his friend Alice to tell him what
public key she uses. Alice hits the "tell someone the name of the
public key" button on her admin user interface and that displays the
nih URI and says "tell this to your buddy". She phones Bob and reads
the nih URI to him.
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Bob types that in to his "manage known nodes" admin application (or
lets that application listen to part of the call), which can
regenerate the ni URI and store that or some equivalent. Then when
Bob's node interacts with Alice's node, it can more safely accept a
signature or encrypt data to Alice's node.
9. IANA Considerations9.1. Assignment of ni URI Scheme
The procedures for registration of a URI scheme are specified in RFC4395 [RFC4395]. The following assignment has been made.
URI scheme name: ni
Status: Permanent
URI scheme syntax: See Section 3.
URI scheme semantics: See Section 3.
Encoding considerations: See Section 3.
Applications/protocols that use this URI scheme name:
General applicability.
Interoperability considerations: Defined here.
Security considerations: See Section 10.
Contact: Stephen Farrell, stephen.farrell@cs.tcd.ie
Author/Change controller: IETF
References: As specified in this document
9.2. Assignment of nih URI Scheme
The procedures for registration of a URI scheme are specified in RFC4395 [RFC4395]. The following assignment has been made.
URI scheme name: nih
Status: Permanent
URI scheme syntax: See Section 7.
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URI scheme semantics: See Section 7.
Encoding considerations: See Section 7.
Applications/protocols that use this URI scheme name:
General applicability.
Interoperability considerations: Defined here.
Security considerations: See Section 10.
Contact: Stephen Farrell, stephen.farrell@cs.tcd.ie
Author/Change controller: IETF
References: As specified in this document
9.3. Assignment of .well-known 'ni' URI
The procedures for registration of a Well-Known URI entry are
specified in RFC 5785 [RFC5785]. The following assignment has been
made.
URI suffix: ni
Change controller: IETF
Specification document(s): This document
Related information: None
9.4. Creation of Named Information Hash Algorithm Registry
IANA has created a new registry for hash algorithms as used in the
name formats specified here; it is called the "Named Information Hash
Algorithm Registry". Future assignments are to be made through
Expert Review [RFC5226]. This registry has five fields: the suite
ID, the hash algorithm name string, the truncation length, the
underlying algorithm reference, and a status field that indicates if
the algorithm is current or deprecated and should no longer be used.
The status field can have the value "current" or "deprecated". Other
values are reserved for possible future definition.
If the status is "current", then that does not necessarily mean that
the algorithm is "good" for any particular purpose, since the
cryptographic strength requirements will be set by other applications
or protocols.
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A request to mark an entry as "deprecated" can be done by sending a
mail to the Designated Expert. Before approving the request, the
community MUST be consulted via a "call for comments" of at least two
weeks by sending a mail to the IETF discussion list.
Initial values are specified below. The Designated Expert SHOULD
generally approve additions that reference hash algorithms that are
widely used in other IETF protocols. In addition, the Designated
Expert SHOULD NOT accept additions where the underlying hash function
(with no truncation) is considered weak for collisions. Part of the
reasoning behind this last point is that inclusion of code for weak
hash functions, e.g., the MD5 algorithm, can trigger costly false
positives if code is audited for inclusion of obsolete ciphers. See
[RFC6149], [RFC6150], and [RFC6151] for examples of some hash
functions that are considered obsolete in this sense.
The suite ID field ("ID") can be empty or can have values between 0
and 63, inclusive. Because there are only 64 possible values, this
field is OPTIONAL (leaving it empty if omitted). Where the binary
format is not expected to be used for a given hash algorithm, this
field SHOULD be omitted. If an entry is registered without a suite
ID, the Designated Expert MAY allow for later allocation of a suite
ID, if that appears warranted. The Designated Expert MAY consult the
community via a "call for comments" by sending a mail to the IETF
discussion list before allocating a suite ID.
ID Hash Name String Value Length Reference Status
0 Reserved
1 sha-256 256 bits [SHA-256] current
2 sha-256-128 128 bits [SHA-256] current
3 sha-256-120 120 bits [SHA-256] current
4 sha-256-96 96 bits [SHA-256] current
5 sha-256-64 64 bits [SHA-256] current
6 sha-256-32 32 bits [SHA-256] current
32 Reserved
Figure 11: Suite Identifiers
The Suite ID value 32 is reserved for compatibility with IPv6
addresses from the Special Purpose Address Registry [RFC4773], such
as Overlay Routable Cryptographic Hash Identifiers (ORCHIDs)
[RFC4843].
The referenced hash algorithm matching the Suite ID, truncated to the
length indicated, according to the description given in Section 2, is
used for generating the hash. The Designated Expert is responsible
for ensuring that the document referenced for the hash algorithm
meets the "specification required" rule.
Farrell, et al. Standards Track [Page 17]

RFC 6920 Naming Things with Hashes April 20139.5. Creation of Named Information Parameter Registry
IANA has created a new registry entitled "Named Information URI
Parameter Definitions".
The policy for future assignments to the registry is Expert Review,
and as for the ni Hash Algorithm Registry above, the Designated
Expert is responsible for ensuring that the document referenced for
the parameter definition meets the "specification required" rule.
The fields in this registry are the parameter name, a description,
and a reference. The parameter name MUST be such that it is suitable
for use as a query string parameter name in an ni URI. (See
Section 3.)
The initial contents of the registry are:
Parameter Meaning Reference
----------- -------------------------------------------- ---------
ct Content Type [RFC6920]
10. Security Considerations
No secret information is required to generate or verify a name of the
form described here. Therefore, a name like this can only provide
evidence for the integrity of the referenced object, and the proof of
integrity provided is only as good as the proof of integrity for the
name from which we started. In other words, the hash value can
provide a name-data integrity binding between the name and the bytes
returned when the name is dereferenced using some protocol.
Disclosure of a name value does not necessarily entail disclosure of
the referenced object but may enable an attacker to determine the
contents of the referenced object by reference to a search engine or
other data repository or, for a highly formatted object with little
variation, by simply guessing the value and checking if the digest
value matches. So, the fact that these names contain hashes does not
protect the confidentiality of the object that was input to the hash.
The integrity of the referenced content would be compromised if a
weak hash function were used. SHA-256 is currently our preferred
hash algorithm; this is why we've added only SHA-256-based suites to
the initial IANA registry.
If a truncated hash value is used, certain security properties will
be affected. In general, a hash algorithm is designed to produce
sufficient bits to prevent a 'birthday attack' collision occurring.
Ensuring that the difficulty of discovering two pieces of content
Farrell, et al. Standards Track [Page 18]

RFC 6920 Naming Things with Hashes April 2013
that result in the same digest with a work factor O(2^x) by brute
force requires a digest length of 2x. Many security applications
only require protection against a second pre-image attack, which only
requires a digest length of x to achieve the same work factor.
Basically, the shorter the hash value used, the less security benefit
you can possibly get.
An important thing to keep in mind is not to make the mistake of
thinking two names are the same when they aren't. For example, a
name with a 32-bit truncated sha-256 hash is not the same as a name
with the full 256 bits of hash output, even if the hash value for one
is a prefix of that for the other.
The reason for this is that if an application treats these as the
same name, then that might open up a number of attacks. For example,
if I publish an object with the full hash, then I probably (in
general) don't want some other application to treat a name with just
the first 32 bits of that as referring to the same thing, since the
32-bit name will have lots of colliding objects. If ni or nih URIs
become widely used, there will be many cases where names will occur
more than once in application protocols, and it'll be unpredictable
which instance of the name would be used for name-data integrity
checking, thus leading to threats. For this reason, we require that
the algorithm, length, and value all match before we consider two
names to be the same.
The fact that an ni URI includes a domain name in the authority field
by itself implies nothing about the relationship between the owner of
the domain name and any content referenced by that URI. While a
name-data integrity service can be provided using ni URIs, that does
not in any sense validate the authority part of the name. For
example, there is nothing to stop anyone from creating an ni URI
containing a hash of someone else's content. Application developers
MUST NOT assume any relationship between the registrant of the domain
name that is part of an ni URI and some matching content just because
the ni URI matches that content.
If name-data integrity is successfully validated, and the hash is
strong and long enough, then the "web origin" [RFC6454] for the bytes
of the named object is really going to be the place from which you
get the ni name and not the place from which you get the bytes of the
object. This appears to offer a potential benefit if using ni names
for scripts included from a HTML page accessed via server-
authenticated https, for example. If name-data integrity is not
validated (and it is optional) or fails, then the web origin is, as
usual, the place from which the object bytes were received.
Applications making use of ni names SHOULD take this into account in
their trust models.
Farrell, et al. Standards Track [Page 19]